In my U.S. Pat. No. 4,481,413, I describe and claim an improvement for autofocus devices used in, for example, video cameras, wherein I provide a totally reflecting strip mirror, rather than a larger partially reflecting mirror used in the prior art, to divert a portion of the radiation from the camera taking lens to the autofocus circuitry for use in determining the correct position of the taking lens. The strip mirror produces a band of radiation which falls across a portion of the radiation detectors that are used in the autofocus system.
In the Norman L. Stauffer U.S. Pat No. 4,309,603, issued Jan. 5, 1982, and assigned to the assignee of the present invention, a system is shown wherein an auxiliary lens is mechanically connected to the camera taking lens and means are provided to move both the taking lens and the auxiliary lens simultaneously. The auxiliary lens is used to provide a path for radiation to the autofocus detectors rather than using any mirror to redirect a portion of the radiation from the taking lens thereto.
The use of an auxiliary lens for autofocus has the advantage that all of the light going through the taking lens reaches the video electronics or film rather than diverting part of it to the autofocus circuitry. The use of the strip mirror also has certain advantages since (1) the detectors no longer have to be shaped in any particular configuration and are thus more easily manufactured (2) the percentage of the surface of the detectors covered by radiation is greater and there is less wasted light and (3) the adjustment of the optics so as to cause the radiation to fall on the detectors is more easily accomplished. One disadvantage of the slave lens system is that the additional lens configuration on the face of the camera is often unsightly. A disadvantage of the strip mirror is that some light is lost on reflection and the mirror physically takes up space in the camera which is often quite limited by the optical and electronic packages.